1. Field of the Invention
The present invention relates to a multiplex communication apparatus for use in a motor vehicle and, more particularly, to a vehicle multiplex communication apparatus which transmits, in accordance with a desired information protocol, each of various control information items and detection information items from control elements and controlled devices via a common bus line provided in a motor vehicle.
2. Description of the Related Art
Motor vehicles having, on the steering wheel or steering column, one or more switches for operating and controlling various mechanisms or devices have been provided. A driver can operate controlled mechanisms or devices provided in a motor vehicle by operating such switches while driving the vehicle. The controlled mechanisms and the operating switches are connected to a plurality of central control units, and information is transmitted over a bus line between the central control units. When some of the operating switches, attached to the steering wheel, for example, are operated by the driver, operation signals indicating the operated states of the switches are collectively supplied to the central control unit also provided on the steering wheel. The collected operation signals are transmitted from the central control unit on the steering wheel to one or more central control units provided on the vehicle body side via a common bus line connected to the central control units. The central control units receiving the operation signals execute the desired operations of controlling the corresponding controlled units according to the sorts and contents of the operation signals. For example, Japanese Patent Laid-Open Publication No. 221322/1993 discloses a vehicle multiplex communication apparatus for performing such control.
The vehicle multiplex communication apparatus disclosed in Japanese Patent Laid-Open Publication No. 221322/1993 has an electrical and mechanical rotary transmission device, a first interface module (central control unit) provided on the steering wheel of a motor vehicle, connected to a plurality of operating switches and controlled with a micro-controller, a second interface module (central control unit) provided on the vehicle body side and controlled with a micro-controller, and a bus line which connects the first and second interface modules. Communication information in the code for the CAN-BUS system is exchanged continuously between the steering wheel side and the vehicle body side in a bidirectional transmission manner.
In general, users select motor vehicles according to their various needs and preferences. Therefore, vehicle operating equipment, accessories and the like as well as a variety of vehicle types are designed so as to largely reflect user's needs and preferences. Also, it is a rather ordinary practice to mount various kinds of equipment or accessories on a motor vehicle or to change already-mounted equipment or accessories at the time of or after delivery of the vehicle to a user according to user's needs or preferences, as well as to manufacture motor vehicles in accordance with various specifications at the production stage. In such a situation, equipment on the steering wheels of motor vehicles is such that an air bag system separately provided is attached at the time of or after delivery to a user, and various operating switches for controlling vehicle equipment are also attached according to user's needs or preferences.
The vehicle multiplex communication apparatus disclosed in Japanese Patent Laid-Open Publication No. 221322/1993 and other known vehicle communication apparatuses of the same kind entail the problem of the number of operating switches (additional control elements) attachable around a steering wheel being limited according to the number of input terminals of the connected interface modules (central control units) and the switching control capacity of the interface modules.
Ordinarily, for multiplex communication in motor vehicles, a bus system having a high transmission capacity, for example, the above-mentioned CAN-BUS is used in order to transmit signals at a high rate. In such a system, however, a central processing unit (CPU) forming a central control unit is necessarily large in scale and high-priced since it processes many signals. Also, if a multiplicity of input devices and switching control units corresponding to the number of switching terminals of the input devices are prepared for adaptation of a multiplicity of operating switches as described above, the manufacturing cost of the apparatus is further increased.
In vehicle multiplex communication, a number of central control units, i.e., CPUs, are employed. If a plurality of different CPUs are prepared in accordance with required functions, the manufacturing cost of the communication apparatus becomes considerably high and the parts management becomes complicated. For this and other reasons, a CPU most excellent in functions is ordinarily used in a shared fashion. Inevitably, such a CPU has a high unit price.
An apparatus for checking the performance of the equipment using the CAN-BUS is large in scale and high-priced. However, it is necessary to use such an apparatus for checking the performance before the steering wheel and other parts are mounted, resulting in a considerable increase in facility investment.
There is still another problem of the vehicle multiplex communication apparatus disclosed in Japanese Patent Laid-Open Publication No. 221322/1993 and other known vehicle communication apparatuses of the same kind. If a multiplicity of operating switches (additional control elements) is mounted around a steering wheel according to user's needs or preferences, the number of signals is correspondingly increased and signals from the operating switches are transmitted over the common bus line simultaneously with ordinary control signals for controlling and operating some controlled units on the vehicle body side or with emergent control signals or the like for controlling the operation of the engine system. Under such a condition, there is a possibility of a delay of transmission of an emergent signal or the like, and, hence a delay of the operation of controlling a controlled unit in the engine or braking system.